It is known that earlier tumor detection leads to the improvement of long-term survival. Therefore, the development of more selective and noninvasive tumor diagnostic techniques is a high priority. Fluorescent imaging has proven to be an efficient tool for preclinical cancer research, antitumor drug discovery and pharmacological developments by providing images of the bio-distribution of fluorescent markers. By tagging regions of interest with tumor-specific fluorescent molecular probes, this technique enables visualization of location and geometries of malignant areas.
The fundamental barriers to optical imaging in tissue are high light scattering, autofluorescence, and high absorption by hemoglobin in the mid-visible band. Use of red and near-infrared light is the most basic step towards improved imaging. Moving to near-infrared wavelengths (700-1100 nm) confers other advantages for imaging mammalian tissues: less background fluorescence is excited, since autofluorescence in tissues is mostly excited by near ultraviolet and blue light; and less autofluorescence interferes, since fluorescence from most mammalian tissues peaks in the yellow and is very low beyond 650 nm. (Ballou B, Ernst L A, Waggoner A S. Curr. Med. Chem., 2005, 12, 795-805; Fabian J, Nakazumi H, Matsuoka M. Chem. Rev., 1992, 92, 1197-1226). The use of near-IR fluorescence improves in many ways the performance of fluorescence-based biological assays. For example, the near-IR fluorescence provides: 1) significant reduction of background autofluorescence; 2) deeper light penetration; 3) minimal photodamage to biological tissue; 4) less sensitivity to the optical properties of the media. A good fluorescent label should have large extinction coefficient, high fluorescent quantum yield and high photostability.
Endoscopy, in particular, colonoscopy and bronchoscopy, is utilized to find abnormal growth and tumors protruding into the lumen. A device, called endoscope, is inserted into a body cavity. Traditionally, endoscopes use a daylight channel, i.e. the observer sees all finding at the wavelength of naturally occurring light.
Lately, newer endoscopes have the ability to utilize several channels, i.e. a daylight channel and one or more additional channels at other light wavelengths. These additional channels are used to monitor either naturally occurring fluorescence or fluorescence of a dye that was either injected into the body or sprayed onto the body cavity surface. One of the possible channels is in the NIR (near infrared) area. The advantage of the NIR area is that the light absorption in the NIR area (usually 600-800 nm) is minimal, and fluorescence can be detected at a depth of a few millimeters to nearly a centimeter beneath the surface of the body cavity. It is believed that this has advantages to detect tumors and lymph node metastases in organs such as colon and lung.
Accordingly, the need exists to further explore the uses of near infrared fluorescence in detecting malignancies during the endoscopic process.